Split asdf/system-registry away from asdf/find-system: the former defines
the internal *defined-systems* table and the search mechanism, whereas
the latter keeps the find-system methods, but also defines a new
define-op operation, on load-asd will call operate.
Reorder dependencies accordingly.
Have forcing come from the plan, so as to not force again actions from
dependencies that were already performed in a subcall to operate.
Accept asdf.asd from the same version, so asdf.asd doesn't get removed
at the very next call to operate (which may now come implicitly
at the least provocation).